In the installation and servicing of air conditioning equipment using freon as the refrigerant, a means of detecting freon gas leakage from the system is a practical necessity. Among the various methods and apparatus heretofore devised for conveniently detecting the leakage of small quantities of freon refrigerant gas into the surrounding atmosphere, negative corona electrical sensing devices have been found to be especially efficient, sensitive, and easy to use. Such instruments are based in theory on the work of Weissler & Mohr, Physical Review, Vol. 72, No. 4, Aug. 15, 1947, pp. 294-297, wherein the effect of substantial lowering of a negative corona discharge current by the introduction of freon in minute quantities into the corona atmosphere is described in detail. In Gazov, Khromatogr. No. 3, 61-6 (1965) (Russ)., V. M. Partispanyan, A. V. Markevich, and S. L. Dobychin describe, in a scientific article titled "Use Of A Negative Corona Discharge For Detecting Gases" how halogen gases in minute quantities remarkably decrease corona discharge current. As an example of a practical freon gas leak detector based on the work of Weissler and Mohr and the Russian scientists, reference can be made to applicant's patent application titled HALOGEN GAS LEAK DETECTOR, Ser. No. 06/304,483, filed Feb. 22, 1981, issued as U.S. Pat. No. 4,488,118. Typically, such halogen or freon gas leak detectors utilize a sensing tip which is projected into the ambient atmospheric zone to be tested or sensed for freon gas contamination indicative of leakage. The sensing tip comprises a small protective shell, constituting the anode electrode, surrounding a point cathode energized by the hand-held body of the instrument to establish a negative corona within the protective shell. Slots or holes provided in the protective shell allow diffusion of the gas molecules into the corona discharge for effecting corona current changes in the presence of freon gas molecules. These current changes activate audio or visual alarms indicating the presence of atmospheric halogen or freon gas contamination or leakage to the operator.
Since the migration of the gaseous atmosphere being sensed through the slots or holes of the protective and corona sustaining shell is slow and often dependent on local air current or wind conditions in the vicinity, the sensing of freon gas contamination is often slow and erratic. In attempts to overcome this disadvantage, small air pumps of one kind or another have been utilized to draw atmospheric air into the protective shell openings and through the corona. This expedient of providing external mechanical means of creating a steady flow of gas to be sampled through the corona is deficient in several respects, principally in that it requires the expenditure of a substantial increase in electrical energy, while at the same time increasing the size of the apparatus, both features of which are undesirable, particularly in hand-held or portable instruments.